1. Field of the Invention
The present invention relates to the transmission of multimedia data, for example, animated or still images, or audio sequences in digital form. The present invention more specifically relates to the transmission of data files made secure by insertion of binary identification or authentication codes or messages. The technique of insertion of binary codes into digital images or, more generally, into data files, is known as watermarking and has had a recent expansion with the rapid development of digital image transmission, especially over the Internet. This technique not only enables protecting digital multimedia contents against possible illicit copies but also preserving these multimedia contents against possible attempts of falsification during a transmission (image tricking or the like).
2. Discussion of the Related Art
An example of application of the present invention is the transmission of digitized images (digital photographs or digitized video sequences) between an image shooting element (for example, a journalist's photographic camera or video camera) and a distant system (for example, a press agency or the studios of a television broadcasting channel).
In this type of application, a double (bidirectional) security is needed. First, the distant shooting element must be sure of the receiver's identity before starting the transmission. Then, the receiver elements must be sure of the origin of the digital images that it receives before broadcasting them. It must also be able to make sure that they have not been modified (tricked) since their transmission.
In practice, the digital shooting device (video or photographic camera) is generally connected to a computer equipped with a modem to transmit the image and/or sound files through a public transmission network (telephone, or the Internet). The receiver is also provided with a modem to receive the data files and with applicative means necessary to their exploitation.
Conventionally, the sender inserts a binary code or message in the images before transmitting them. A so-called watermarking algorithm is used to code or position in the images the messages to be inserted therein. This watermarking is generally performed in a way imperceptible to human eye. A read or extraction algorithm is used on the receive side to restore the messages formed, according to the application, of identification, authentication codes, or the shooting characteristics, etc.
In the case of animated sequences, all the images or part of them then include an insertion (for example, an author identification code is reproduced in all images or in part of them). In this example, it is besides possible for the content of the respective codes to be linked to the sequence to detect a possible image insertion or suppression.
Generally, watermarking algorithms modify either certain pixels in the image, or certain coefficients in an image transform (Fourier transform, discrete cosine transform, etc.). The pixels or coefficients to be modified are generally selected by using a watermarking key specific to the user (the coder). The modifications are, for example, an inversion of coefficient pairs so that their value respects a relation depending on the value of the bit to be hidden, or a mere addition of the message to be hidden previously modulated by a Gaussian white noise. In certain cases, the watermarking visibly modifies the original image.
Without the watermarking key, the receiver element cannot recover the message. Further, if the image is visibly modified by the watermarking, it then cannot even recover the original image. Further, if it has the key, it can detect that the image that it receives has a strong risk of having been falsified if it does not recover the expected message.
A disadvantage of conventional transmission methods is that the sender is not able to make sure, before transmission, that it really is in communication with a given receiver. Indeed, a pirate receiver may pretend to be the receiver of the images or intercept a transmission which is not intended for it.
U.S. Pat. No. 5,544,245 discloses an example of a system providing a mutual authentication key based on a symmetrical cyphering. The purpose of this document is to obtain a common key on the transmitter and receiver sides. This document only aims at providing an authentication or cypher key and not at marking data to be transmitted from a transmitter to a receiver.